1. Field of the Invention
The present invention relates to a semiconductor memory device, more particularly, to a semiconductor memory device using a transistor whose emitter current or source current has negative differential resistance characteristics. The transistor used for this semiconductor memory device is, for example, a Resonant-Tunneling Transistor (RTT), which has a resonant-tunneling barrier for injecting carriers.
2. Description of the Related Art
Recently, a Resonant-Tunneling Transistor (RTT) having a resonant-tunneling barrier for injecting carriers has been provided. This resonant-tunneling transistor includes a Resonant-Tunneling Hot-Electron Transistor (RHET) and a Resonant-Tunneling Bipolar Transistor (RBT) and the like, and has negative differential resistance characteristics in the emitter current of the transistor, and a high speed operation. Furthermore, an FET of the resonant-tunneling transistor type, which has a resonant-tunneling barrier for injecting carriers and negative differential characteristics in the source current of the FET, has been studied and developed in recent years. In these RTT elements, the emitter current (or the source current) relative to the base-emitter voltage (or a gate-source voltage) has N-shaped characteristics which increase, decrease, and then again increase. In addition to the above RTT elements, a Real Space Transition Transistor is known as a transistor having negative differential resistance characteristics in the source current thereof.
Incidentally, in accordance with the requirements of a high speed operation and a high integration, it is required that a configuration of a basic cell becomes simplified. Namely, for example, in a general static random access memory (SRAM), a flip-flop (basic cell) is constituted by a pair of crossconnected transistors, and a plurality of resistors or diodes, and one of two different operating states is selectively maintained. However, in this prior basic cell, a plurality of elements are required. For example, depending on the microscopic art of miniaturizing transistors and the like, it is difficult to satisfy the requirements of high speed operation and large scale integration in recent years. Therefore, a semiconductor memory device (for example, SRAM), which has fewer elements and selectively maintains one of two different operating states by using a more simplified basic cell, is required.
In order to satisfy the requirements of high speed operation and the large scale integration, a semiconductor memory device which uses an RTT having a resonant-tunneling barrier for injecting carriers, has been provided by the present applicant in Japanese Unexamined Patent Publication (Kokai) No. 63-269394 (Japanese Patent Application No. 62-103206). However, in the semiconductor memory device using the RTT of JPP '394, for example, a base current I.sub.B against a base-emitter voltage V.sub.BE should have negative differential resistance characteristics, a collector current I.sub.C should have characteristics of largely flowing after appearance of the negative differential resistance characteristics, and thus the RTT used for the above semiconductor memory device should be produced to purposely decrease its current gain. Therefore, a variation of the design of the RTT becomes small in size and a high speed operation of the RTT decreases. Furthermore, when the RTT is used for a logic element (for example, an exclusive NOR element), in the above semiconductor memory device, an RTT used for the logic element is not produced by the same production processes as used to produce the RTT used for the semiconductor memory device, since a collector current of the RTT used for the memory device should have negative differential resistance characteristics and a collector current I.sub.C of the RTT used for the logic element should flow after appearance of the negative differential resistance characteristics of a base current I.sub.B (which is described later in detail).